1. Field of the Invention
The present invention relates to a method of detecting a pneumatic pressure drop of a tire provided for a four-wheel vehicle, and a tire pressure drop detecting device for carrying out this method.
2. Description of the Related Art
As one of safety devices for a four-wheel vehicle such as an automobile, truck, etc., tire pressure drop detecting devices have recently been proposed, and some of them have been put to practical use.
A tire pressure drop detecting device has been developed because its importance is recognized due to the reason shown below. That is, when the pneumatic pressure drops, the temperature of the tire increases due to deflation. When the temperature becomes high, the strength of a polymer material used for the tire is lowered and the tire is likely to burst.
Normally, even if the tire is deflated by a pressure of approximately 0.5 atm, a driver is often unaware of the deflation, so that a device capable of detecting the deflation has been desired.
One method of detecting a tire pressure drop, for example, is a method of utilizing a difference in the respective rotational angular velocities F.sub.1, F.sub.2, F.sub.3 and F.sub.4 (referred to as a "rotational angular velocity F.sub.i " hereinafter) of four tires W.sub.1, W.sub.2, W.sub.3 and W.sub.4 provided for the vehicle. The tires W.sub.1 and W.sub.2 correspond to right front and left front tires, and the tires W.sub.3 and W.sub.4 correspond to right rear and left rear tires, respectively. The tires W.sub.1, W.sub.2, W.sub.3 and W.sub.4 are referred to as a "tire W.sub.i " hereinafter.
When the detected rotational angular velocities F.sub.i are the same, then dynamic loading radiuses (value obtained when a travel distance of the vehicle per unit time is divided by the product of the number of revolutions of the tires and 2.pi.) are the same.
On the other hand, the dynamic loading radius of the tire W.sub.i varies depending on the change in pneumatic pressure of the tire W.sub.i. That is, when the pneumatic pressure of the tire W.sub.i drops, the tire W.sub.i contracts. As a result, the dynamic loading radius becomes smaller than that in case of a normal inner pressure. Accordingly, the rotational angular velocity F.sub.i of the tire W.sub.i whose pneumatic pressure drops becomes larger than that at a normal internal pressure. Therefore, the pressure drop of the tire W.sub.i can be detected based on the difference in rotational angular velocities F.sub.i of the tires.
One embodiment of a method of determining the pressure drop of the tire W.sub.i based on the difference in rotational angular velocity F.sub.i is as shown in the following equation (1) (e.g. see Japanese Laid-Open Patent Publication Nos. 63-305011 and 4-212609). ##EQU1##
For example, when it is assumed that the dynamic loading rolling radiuses of the tires W.sub.i are the same, the rotational angular velocities F.sub.1 are the same (F.sub.1 =F.sub.2 =F.sub.3 =F.sub.4). Accordingly, the judged value D becomes 0. Then, threshold values D.sub.TH1 and D.sub.TH2 are set (provided D.sub.TH1, D.sub.TH2 &gt;0). When the condition shown in the following expression (2) is satisfied, it is judged that a tire W.sub.i whose pneumatic pressure drops is present. When this condition is not satisfied, it is judged that a tire W.sub.i whose pneumatic pressure drops is not present. EQU D&lt;-D.sub.TH1 or D&gt;D.sub.TH2 ( 2)
However, when the vehicle is traveling at high speed, the judged value D is sometimes lowered in spite of the existence of a tire whose pneumatic pressure has dropped. In this case, the judged value D does not satisfy the expression (2) and it is likely to be erroneously judged that all tires W.sub.i have a normal inner pressure.
FIG. 12 is a graph showing a change of the judged value D, which is calculated when the pneumatic pressure of any one of four tires W.sub.1, W.sub.2, W.sub.3 and W.sub.4 drops when the vehicle is traveling on a flat road, with the velocity of the vehicle. As is apparent from FIG. 12, the judged value D decreases as the velocity of the vehicle increases and becomes generally 0 (zero) when the velocity of the vehicle is about 200 km/hour.
The cause of the decrease in judged value D is as follows. That is, it is known that a grip rate of the tire W.sub.i whose pneumatic pressure drops is larger than that of the tire W.sub.i whose pneumatic pressure is a normal inner pressure. The tire W.sub.i normally rotates with a slight slip. Regarding the tire W.sub.i whose pneumatic pressure drops, however, the slip amount decreases in an amount corresponding to the increase in grip rate. The decrease in slip amount becomes severe as the velocity of the vehicle increases, to cancel the decrease in the number of revolutions due to contraction of the tire W.sub.i. As a result, the phenomenon as shown in FIG. 12 arises.
It is considered to apply the technique disclosed in Japanese Laid-Open Patent Publication No. 7-125512 as the technique to cope with the above phenomenon. In the technique disclosed in this gazette, the judged value D is calculated, and then the calculated judged value D is subjected to a velocity correction.
More specifically, in the technique disclosed in the above gazette, a correction factor corresponding to the velocity of the vehicle is previously determined. Specifically, the judged value D must be drastically increased as the velocity of the vehicle becomes higher. Therefore, there is required a correction factor wherein an increase rate becomes higher as the velocity of the vehicle becomes higher. The judged value D is corrected so that the judged value D is multiplied by the reciprocal of the correction factor. As a result, the judged value D is increased to a value large enough to represent the fact that the pneumatic pressure drops. Accordingly, the judged value D in a case that the tire pressure drops satisfies the above expression (2). Therefore, an erroneous detection caused by the decrease in judged value D can be prevented.
By the way, the above velocity correction is conducted not only when the pneumatic pressure of the tire W.sub.i drops but also when all tires W.sub.i have a normal inner pressure. In this case, no problem arises if the judged value is always 0 (zero). Because, even if 0 is multiplied by any numeric value, the resultant is always 0.
However, actually, the judged value D does not necessarily become 0, even if all tires W.sub.i have a normal inner pressure. Because, the rotational angular velocity F.sub.i of each tire Wi varies depending on the traveling state of the vehicle and state of the road surface. In this case, the variation of the rotational angular velocity W.sub.i is promoted when the judged value D is subjected to the velocity correction by which the judged value D is increased. As a result, it is likely to be erroneously detected that there is a tire W.sub.i whose pneumatic pressure drops.
On the other hand, the increase in velocity of the vehicle means an increase in driving torque exerted on the tire W.sub.i. That is, the decrease in judged value D in spite of the existence of the tire W.sub.i whose pneumatic pressure drops is caused by the fact that a large driving torque is actually exerted on the tire W.sub.i.
Examples of traveling under the condition that a large driving torque is exerted on the tire W.sub.i include traveling on an upward slope, in addition to the traveling at high speed. Accordingly, the decrease in judged value D is observed due to the same reason as that described above when the vehicle is traveling on the upward slope, even when the vehicle is traveling at low speed.
It is considered to subject the judged value D to the above-described velocity correction so as to cope with the above problem. However, the correction factor in the above velocity correction is required so as to increase the increase rate as the velocity of the vehicle becomes higher and, therefore, the judged value D hardly increases when the vehicle is traveling at low speed. Accordingly, it is likely to be erroneously detected that all tires W.sub.i have a normal inner pressure in spite of the existence of a tire W.sub.i whose pneumatic pressure drops.
In such way, when the vehicle is traveling under the condition that a large driving torque is exerted on the tire W.sub.i, erroneous detection is sometimes conducted because the velocity correction is made, and the velocity correction is sometimes meaningless for avoiding an erroneous detection.